1. Field of the Invention
The invention pertains to a device for limiting the prestress of a control spring, specifically a control spring of a pressure control valve, a pressure relief valve, a pressure scale, or similar devices, wherein the control spring is located in a spring housing and is supported on a spring plate which is screwed onto an adjustable spindle that is provided with an outer thread and can be turned by means of a handwheel and is secured in the spring housing against turning, but is held so it can be displaced axially, and wherein the control spring is combined with an end stop for the spring plate which limits its adjustment for the prestressing of the control spring.
2. The Prior Art
In control engineering it is well-known that the size of the controlled value, e.g., the pressure of a pressure control valve, is determined by the force exerted by the control spring. The desired value is set by changing the prestress of the control spring. In that regard, it is normally possible to prestress the spring to an extent such that the controlled value attains its maximum value, which in the case of a pressure regulator means that the controlled pressure matches the inlet pressure which is being supplied. In many cases, a limiting of the controlled value to a lower figure is necessary for safety reasons.
In order to make it possible to limit the controlled value, it is well-known in the case of devices as described above that a fixed limit stop which limits the adjustment area is provided on the adjustable spindle, by means of which the spring plate for the prestressing of the control spring can be adjusted, or else it is provided on another screw for adjusting the prestress. In series production, this limit stop is often mounted at a location at which it prevents the control spring being pressed to the blocking point. For applications in which a specific, lower value is desired, the limit stop must be placed at the appropriately matched location during production.
Two designs are known for pressure control valves. In one design the spindle length and the fixed limit stop are already configured or arranged in terms of their design in such a way that the control spring can be prestressed only to the predetermined extent. This design makes possible normal, series-production assembly. However, this results in a large tolerance in the limit value achieved by the adjustment, since length tolerances of the adjustable spindle, spring pressure tolerances, and tolerances in the effective membrane diameter add to one another and collectively have an effect on the limit value.
With the second design the limit stop is formed by means of a limit stop nut which, after the desired prestress has been adjusted, is screwed onto the adjustable spindle until it makes contact with the spring plate. Doing this makes it possible to more accurately limit the pressure. However, this limitation can also be carried out only at the factory, and cannot easily be changed afterwards. In addition, doing this also requires partial disassembly of the device after it has been adjusted in order to secure the limit stop nut. Subsequent to that, the pressure regulator must be checked once again, because it is possible that the desired prestress has been changed by the assembly work.
The invention performs the task of improving the previously known devices for limiting the prestress of a control spring in such a way that the limitation can also be simply and accurately adjusted to a desired value by the end user of the device in question without difficult assembly work.